I did that once. Going through the rEvo checklist I had a glitch between opening the DIL and getting to the O2 turn on. Was on the surface and saw the PPO2 dropping, spit out the loop and breathed some air.
Changed my checklist so opening O2 was WAY up on the list. As in open DIL, check ADV, open O2...
I would have thought that everyone is taught to turn on your oxygen before anything else. Both units I'm certified on oxygen its absolutely on before you put the unit on your back. That should be standard. Scary that it's not.